Strongyles are the really bad kids on the block. They top the most-unwanted list on pretty much any list of equine parasites.
They pose a serious threat to horse health.
In fact, if your horse is unwell or dies because of a parasite burden, there’s a pretty good chance that strongyles will be responsible.
They are worm-like in appearance and are white or red in colour, ranging from around 2mm in length up to about 50mm.
There are dozens of strongyles but they are lumped into two main categories — the large strongyles (strongylinae) and small strongyles or small redworms (Cyathostominae), of which there are about 50 different kinds that affect horses. About 10 of these are considered common.
While the large strongyles are the monsters of the family, small strongyles are still visible to the naked eye.
Large strongyles comprise bloodworms (Strongylus vulgaris) and large redworms (Strongylus equinus and Strongylus edentatus).
They are all members of the wider nematode family. There are males and females, with the females capable of producing vast numbers of eggs.
A horse infected with strongyles can be passing millions of eggs a day in their dung, laying the foundation for the next life cycle.
While hanging out in a horse’s insides is a pretty pleasant place to be if you’re a strongyle, very significant stages of their life cycle must occur outside the horse.
Let’s follow one of those strongyle eggs: Once produced by the female adult strongyle in the horse’s gut, it has a one-way ticket out of the horse’s gut, eventually ending up in a pile of dung with countless thousands of its mates.
Life outside a horse is a very different place and temperature will play a very big part in the rate at which the egg can hatch and develop.
The temperature must be in the range of 7 deg Celsius to 30 deg Celsius in order for the eggs to hatch. In most temperate areas, that pretty much rules out winter.
If temperatures are to their liking, the eggs will hatch quite quickly into first-stage (L1) larvae and get sustenance from the manure. They molt into second-stage (L2) larvae and finally the L3 stage. They are then – and only then – capable of re-infecting a horse.
In ideal conditions, a strongyle egg can go through the three larval stages in three days. If conditions are more marginal, the process could take several weeks.
The third-stage larvae are very different from the first two stages. They are tightly enveloped in a tough membrane and don’t even have a mouth. These larvae are intent on just one thing: surviving long enough to infect another horse.
Without the ability to feed, third-stage strongyle larvae are literally in a race against time and the vagaries of weather. The warmer the temperatures, the faster their metabolism and the quicker they use up their energy store and die.
However, in wintry or freezing conditions they use hardly any energy and can hang on for months. So while a good frost might kill some of the less resilient strongyle eggs in a pasture, those same conditions are providing the infective third-stage larvae with that most valuable of commodities – time – to infect a horse.
The tough membrane will also protect the strongyle from drying out, although, as mentioned above, heat will see them use up their energy reserves and die considerably quicker.
They hang out on blades of grass in the hope that a passing horse will ingest it.
What does this mean for horse owners?
Strongyle eggs rapidly turn into infective larvae during warm summers, but the larvae have a shorter lifespan. However, larvae that reach their final infective stage on the cusp of winter can hang in for months.
Both large and small strongyles follow a similar life cycle in pasture, but their behaviours inside a horse are very different.
If you think that large strongyles spend all their time hanging out in the gut, you’d be very wrong. In fact, they can spend months migrating through parts of the horse, including important organs such as the liver.
Take Strongylus vulgaris, for example, which home in on the arteries that feed the gut, causing damage to the artery walls. Or Strongylus edentatus, which targets the liver. Yet another kind — Strongylus equinus — has a liking for the pancreas as well as the liver.
All roads, as they say, lead to Rome — or in the case of large strongyles, back to the gut.
Their journey over, they mature and set about producing eggs which take that familiar exit route through the intestine to begin the life cycle all over again.
You don’t have to be a parasitologist to realise that the migrating habits of large strongyles can do serious damage. Firstly, there are classic symptoms of worm infestation: weight loss, a dull coat, poor appetite, digestive upsets — either diarrhoea or constipation — and a tell-tale pot belly.
They can cause internal bleeding and inflammation in a variety of places and even rupture the mesenteric artery, usually with fatal consequences.
They can cause blood clots, called a thrombus, which have the potential to break loose and cause damage by blocking off blood supply to tissues, most often a section of the large intestine.
They can cause anaemia, damage artery walls, trigger liver problems and even colic. In short, your horse can do without large strongyles.
Small strongyles are focused on the gut and leave the other major organs pretty much alone. This means they are classified as non-migratory worms. However, they do have some very nasty habits that can make them difficult to control.
Strongyles tends to head for the large intestine and burrow into the intestinal lining. The horse’s body retaliates by forming scar tissue that envelops each larva. Behind such a protective barrier, these so-called encysted strongyles are remarkably resilient.
These little nodules are usually filled with blood and it is probable that the larvae feed on the blood.
Until comparatively recently, drenches struggled to make a dent in the population of encysted strongyles.
Safely entombed, they go through several larval stages before finally maturing into adults and producing vast numbers of eggs, which pass out of infected horses in their dung.
Encysted strongyles do not all develop at the same rate and this appears dependent upon the degree of infestation.
In heavily infected horses, encysted strongyles are capable of waiting for more than two years before finally maturing.
Small strongyles can cause serious problems. There might be some local inflammation when the infection first occurs, but horses show virtually no signs of infection once the larvae are encysted in the intestinal lining.
However, when the larvae mature and emerge from the cyst the localised inflammation can be quite severe. A horse can lose its appetite and suffer from diarrhoea.
Bleeding from the areas of inflammation can lead to anaemia and there is a very serious complication from the emergence of large numbers of encysted strongyles called larval cyathostomosis, resulting in severe irritation of the gut and a serious reduction in normal gut action, which can lead to colic and death.
With small strongyles, it is the larval stages that do the most harm to a horse, not the adults.
Strongyles, we have learned, hardly play fair. It’s enough to get us heading straight for the drench. But controlling strongyle infestation is not that clear cut.
While drenches may proclaim their broad-spectrum properties and claim to be effective against “strongyles”, what does that really mean?
In some cases, the effectiveness the label talks of will relate only to the adult egg-laying stage.
It is entirely possible that a particular drench will be ineffective against encysted small strongyles, or large strongyles during their meandering through your horse’s liver or pancreas.
Let’s deal with these wandering large strongyles first. Ivermectin given at a single dose of 0.2 milligrams per kilogram will deal them a killer blow, as will moxidectin at a single dose of 0.4 milligrams per kilogram. Fenbendazole will also work, but a horse requires 10 milligrams per kilogram of body weight for five days in a row.
Encysted small strongyles present a special challenge and will survive most drenches, including even ivermectin.
Only two drenches are known to do the trick against these encysted larvae – moxidectin at the above-mentioned dose rate of 0.4 milligrams per kilogram, and the five-day course of fenbendazole at the rate of 10 milligrams per kilogram of body weight each day. However, fenbendazole can be expected to work against these encysted larvae only if there is no resistance against it — and resistance is everywhere.
It should be noted that moxidectin is not effective against encysted small strongyles when they first enter the intestinal lining, known as the early third-stage larva (EL3), but is effective against the later encysted stages.
Strongyles clearly are a formidable foe. We need to be on our game if we want to keep the upper hand.
We can take away a few important lessons from our knowledge of strongyles. We know that we can check the faeces of horses for eggs which can indicate the presence of adult, egg-laying strongyles.
While the life cycles of different strongyles vary considerably within the horse, their life cycles in pasture are remarkably similar.
It is here we get our first insights into how pasture management strategies can impact directly on the rate at which horses become infected. Clearly, prompt removal of manure from the pasture will have a real impact in containing strongyles. If conditions are warm, this needs to be done within three days to prevent the larvae from reaching an infective stage.
We also know that horses are at the greatest risk of infection during the winter months, when the infective third-stage larvae are able to survive the longest. This means we can be strategic in the timing of our use of drench.
Strongyles are a formidable enemy and in their encysted stage can be difficult to kill. It’s just as well that every other equine parasite isn’t as resilient.
» Next: What’s so scary about ascarids?
First published on Horsetalk.co.nz in February, 2009